1. Field of the Invention
The field of the invention relates to arrangements and methods of retaining the inboard end of composite propeller blades in a hub, and is more particularly, but not exclusively, concerned with mounting arrangements for aircraft propeller blades of a composite construction.
2. Description of Related Art
Aircraft propellers comprise two or more blades mounted at their inboard end to a hub fixed with an engine shaft. Propellers having a variable pitch are arranged so that their pitch can be adjusted during use, or can be adjusted when stationary on the ground. In such propellers, the inboard or root ends of their blades have a circular cross-section so that they can be clamped (in the case of a ground-adjustable pitch) or retained in the hub in circular, rolling-element bearings (in the case of variable-pitch propellers) to allow for the blade pitch to be changed. Variable-pitch aircraft propellers are used to adjust the power absorption of the propeller from the driving engine by changing blade pitch, and some may also be “feathered” to produce little or no thrust and low drag, or produce reverse thrust for use on the ground, and have been in use almost since the advent of powered flight. Propeller blades have been manufactured in a variety of materials, but all variable pitch blade roots have a metal root end to interface with the retaining bearing. The metal blade root is either integral with the blade aerofoil if it is metal, or fixed to the blade aerofoil if it is not metal. The circular-section, metal root-end may also provide a circular dynamic sealing feature to contain any lubricating fluid within the hub as the blade pitch changes.
The cross-section of conventional composite aircraft propeller blades, and solid metal blades on variable-pitch propellers, changes gradually from an aerodynamic aerofoil shape to a circular or near-circular cross-section as the blades enter the blade root. This is referred to as the blade transition zone, and is necessary for structural reasons. It has the effect of increasing the aerodynamic drag of the inboard end of the blade, which is sometimes partially mitigated by moulding or otherwise attaching a non-structural fairing around the structure.
As well as producing and reacting the thrust loads of the propeller, propeller blades are subject to large centrifugal loads due to their rotation. They are also exposed to high-frequency vibration loads both from torque variations from the engine, especially if it is a direct-drive reciprocating piston engine, and from aerodynamic effects associated with airflow entering the plane of rotation of the propeller at an angle offset from the axis of rotation, known as “IP” loads. Blades have to endure these loads over a very large ambient temperature range encountered on the ground and in flight. The detachment of a propeller blade from a propeller during operation is a hazardous and potentially catastrophic occurrence, and accordingly the propeller blade and hub are classified as “critical parts” on aircraft. Therefore, when a propeller blade aerofoil is manufactured from a material other than the metal used for the blade root, the fixing of the one to the other is a critical design feature called upon to work in a difficult environment. Composite blades offer a significant weight reduction compared with metal blades, and many different means of fixing the composite blade safely to the metal root member are known.